drm/amd/display: make PSR static screen entry within 30 ms

[Why]
With different refresh rate panels, the PSR entry/exit time is
different since it is dependent on 2 frame entry time today

[How]
Make static screen num frame entry time to be calculated
such that entry time is within 30 ms instead of fixed num
frames.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>